In the processing of thin, flat articles such as semiconductor wafers, the articles are frequently heated to elevated temperatures for processing in sealed vacuum chambers. Thermal treatment of the wafers is sometimes a necessary or incidental component of a coating or etching process and at other times is the essence of the process itself, such as in rapid thermal processing, annealing or degassing.
In sealed chambers such as vacuum processing chambers, where semiconductor wafers are usually processed, various temperature schemes have been employed. Thermcouples, for example, attached to or are held in contact with the wafer, or mounted in the wafer support are slow to respond to temperature changes and are often introduce a source of wafer contamination. Pyrometers, on the other hand, may avoid direct contact with the articles being processed, but have the disadvantage of being sensitive to the emissivity of the material of which the wafer or coatings added to it are made. Emissivity is also subject to change during processing.
Optical methods for indirectly measuring the temperature of an article by measuring its thermal expansion have been proposed. These optical methods provide an advantage of being related only to the coefficient of thermal expansion of the material that forms a structural core of the wafer, which is constant and can be reliably determined. Such proposed methods have, for example, included the formation of images of the article, and have required equipment such as optical sensors placed close to the article where their accuracy is affected by the process within the chamber. Such methods are in practice used off-line, and where so used, atmospheric refraction can detract from the accuracy of the measurement process.
Where articles such as semiconductor wafers are mounted on a support within a sealed chamber for processing, particularly where wafers are processed in batches, as is often the case in degassing or other thermal treatment processes, misalignment or distortion of the support may alter the wafer position and render optical temperature determination methods unreliable.
Accordingly, there is a need for a temperature determination method and apparatus particularly useful for the processing of semiconductor wafers in sealed chambers, and more particularly useful in processes wherein multiple wafers are simultaneously processed, to accurately and instantaneously determine the wafer temperature. Further, there is a need to provide such a temperature determination method which is less affected by sources of measurement error such as the accuracy and the positioning of the article, changes in article emissivity or atmospheric distortion of the measured medium.